From Seo Wiki - Search Engine Optimization and Programming Languages

Hickson has stated that development will begin when some combination of three of the four major rendering engines (namely Trident, Gecko, WebKit, and Presto) have builds (web browsers) that pass the Acid3 test, and will be finished and announced after 3 of the four major rendering engines have announced that they have fixed all the bugs found by Acid3. As of January 2010, there are generally available versions of browsers based on WebKit and Presto with a score of 100/100, while the best non-beta browser scores for Gecko and Trident are at 94 and 20. It has been suggested that Acid4 will be primarily focused on Scalable Vector Graphics, CSS3 and mixed namespaces.[1]